Before crude oil can be used as a fuel or chemical feedstock, it must first be separated into different fractions, which are typically classified according to their viscosity. These must then be cleaned up; for example, to remove sulfur. Heavier fractions often contain more sulfur in the form of polycyclic aromatic sulfur heterocycles, which are difficult to remove with existing catalytic processes. To improve these processes, it is important to sift out knowledge about the molecular structures containing the sulfur from the highly complex overall mixture. Panda et al. have used a range of different ionization techniques, in conjunction with chromatography and mass spectrometry, to characterize the types and classes of sulfur-containing polyaromatic molecules in a crude oil fraction. Each ionization technique favors some molecules but discriminates against others, thus providing different windows for studying the system. Use of any one technique in isolation might therefore bias the results, but used in conjunction, they can elucidate the complex composition of crude oil and may inform the development of better sulfur-removal processes. — JFU